1. Field of the Invention
This invention relates to the field of medical devices, more particularly, to coatings for devices such as stents.
2. Description of the Background
In the field of medical technology, there is frequently a necessity to administer drugs locally. To provide an efficacious concentration to the treatment site, systemic administration of medication often produces adverse or toxic side effect for the patient. Local delivery is a preferred method in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Thus, local delivery produces fewer side effects and achieves more effective results.
One commonly applied technique for local delivery of a drug is through the use of a polymeric matrix. A polymer impregnated with a drug can be formed into particles or can be coated on implantable medical devices such as stents. Subsequent to the implantation of the particle or the device, the drug slowly elutes from the polymer. A variety of well known polymers have suitable biocompatible properties which allow the polymers to serve as a suitable host for local drug delivery. A selected group of these polymers can also form a film layer or a coating for implantable devices such as stents. One example of a polymer that serves the dual function of being very biocompatible and capable of forming a coating for devices is a copolymer of ethylene and vinyl alcohol, also known as poly(ethylene-co-vinyl alcohol) or EVOH. Poly(ethylene-co-vinyl alcohol) is also known under the trade name EVAL and is distributed commercially by Aldrich Chemical Company of Milwaukee, Wis. EVAL is also manufactured by EVAL Company of America of Lisle, Ill. Other polymers which can be used to coat stents include a copolymer of ethylene with acrylic acid (EAA) and a copolymer of ethylene with glycidyl methacrylate (EGMA).
EVAL is a product of hydrolysis of ethylene-vinyl acetate copolymers. EVAL may also be a terpolymer and may include up to 5% (molar) units derived from styrene, propylene and other suitable unsaturated monomers. EVAL can be described as being hydrophobic and thus is essentially insensitive to moisture. EAA and EGMA, likewise, are hydrophobic and relatively impermeable to gases. The ethylene fragments of EVAL, EAA and EGMA provide hydrophobicity and barrier properties, while functional fragments of each copolymer (hydroxyl groups, carboxyl groups, and glycidyl groups, respectively) provide at least limited solubility in organic solvents.
While EVAL, EAA and EGMA are inert and biocompatible polymers which are quite suitable for use as a drug delivery matrix, and more particularly when used in conjunction with medical devices, some of the properties of these polymers can be improved. In particular, the polymers are prone to protein fouling, which may significantly inhibit the polymers' life time in vivo efficacy.
There is a need for polymeric carriers suitable for the delivery of drugs, and more particularly for coating medical devices used as a means for drug delivery. Suitable characteristics of the polymeric materials should be significantly impermeable to oxygen, high degree of hydrophobicity and long term biocompatibility with minimum protein fouling effects.